News from studies @ Louvain K. Piotrzkowski (UCL)

1. News from studies @ LouvainK. Piotrzkowski (UCL) Fast simulation of forward proton detection J. de Favereau & X. Rouby Luminosity with very forward e+e- pairs in CASTOR D. Bocian (CERN/Krakow) & KP

2. Roman Pots Q D2 ~75 m IP ~160 m D1 Measuring (very) forward protons • To good approximation beam protons move independently in horizontal and vertical planes • Particle point-to-point transfer can be computed using transport matrices (X=MX0) or, equivalently optics functions b, F and D x = x*(b/b*)sinF + qx*(bb*)cosF + DDE/E = x Horizontal plane:

3. Measuring forward protons II • ‘Magic’ location ~ 220 m from IP: phase advances Fx~p, Fy~p/2 angles and displacements are ‘decoupled’; • b function has a local minimum detector can be close to beam • (bx :0.5  8-12 m ; by :0.5   500 m) • Energy loss x results in a horizontal displacement Dx (with a large dispersion D 100 mm) sx*=sy*=16 (8) mm sqx*=sqy*=32 (16) mrad I.P. Detector@220 m Intrinsic smearing qx* 4 qx dqx  8 (4) mrad qy*70 Dy [mrad/mm] dDy  500 (250) mm x Dx /D dDx  60 (30) mm Q2 pT2  E2 (1-x)[ (qx*)2 +(qy* )2]

4. First run of fast MC • Run MC: • smear vertex • transport scattered p • gaussian detector • smearing (10 mm) • reconstruct variables = 10  from beam 100 mm 2 mm Example: 200 GeV photons with Q2=2 GeV2

5. Beam spot/closest approach/... • Beam spot at 220 m for low-b is asymmetric: 30 mm500 mm (Different segmentation requirements for x and y coordinates: strips with different pitch?) • Very high event rates in small area: very large fluences • (Horizontal) very close approach possible: 1 mm eq. to > 15 s ! Will continue the development: include all beam-line elements, apertures, non-gaussian smearings

6. pp Luminosity Excellent QED process for luminosity monitoring pp  pp e+e- Electrons are in forward range Cross section ~barns Tracking important! EM calorimetry essential Can get the luminosity to 1-2%? So far > 5% from W,Z production, (or via optical theorem at very low luminosity)  small angles of few mrad  high rates of few kHz background! s varies strongly with pair energy

7. Main features of two-photonprocess The QED diagram and the kinematical variables:

8. Large cross-section + clear signatures electron energy: i=15GeV photon energy: 1, 2me2/ (5GeV, 50eV) pair mass: W2me (< 20MeV) pair pT: PTme (< 20MeV) single electron pt: pT(1),pT(2)me acoplanarity angle:   PT/pT

9. Forward e+e-pairs : (Some) Background diagrams Feynman diagrams of possible background

10. Forward e+e-pairs in pp Signal and background simulations Most important distributions for the signal and the backgrounds A. Shamov & KP ‘99:

11. Forward e+e-pairs Cross section values has been obtained (b) -signal, dd-Dalitz decays, bs-bremsstrahlung, total=bs+dd

12. Forward e+e- pairs in ion collisions at the LHC Recent studies: Consider use for luminosity monitoring in ion collisions at the LHC Use modified LPAIR (ME generator of lepton pairs by Vermaseren), i.e. Introduce new type of projectile, so far only elastic (form-factor) High rates for e+e- pairs within the CASTOR acceptance Excellent candidate for the luminosity measurement for ion collisions

13. Pair distributions in CASTOR

14. CASTOR simulation So far only ideal case assumed: geometrical acceptance + gaussian smearing (no dead material) -DE = 20%E and Dx = 0.5 mm Need ORCA for realistic simulation of critical resolutions in pair pT and acoplanarity (need to quantify effect of solenoid field in CMS!) Continue studies on the pp case with CASTOR